1. Field of the Invention
This invention relates generally to a semiconductor optical modulator and, more particularly, to a semiconductor traveling wave optical modulator that employs diffraction gratings adjacent to the waveguide to slow the optical wave to a speed that matches the speed of the electrical wave in the transmission line.
2. Discussion of the Related Art
Electro-optical (EO) modulation devices are known in the art that use an electrical signal to frequency modulate an optical signal to impress information on the optical signal. One class of modulation devices of this type include semiconductor quantum well modulators that employ PIN semiconductor device architectures where the optical wave propagates down a waveguide defined within the intrinsic layer in the device. An RF signal is applied to electrodes in contact with the P and N layers to provide the modulation voltage, where the electrodes define an RF transmission line.
Low modulation voltages (sub-volts) are important to the performance of electro-optical quantum well modulators of the type being discussed herein. Semiconductor quantum well modulators have the potential to achieve the low modulation voltages that are desirable for system applications. These modulators can reach very low operation voltages because of the epitaxial layer structure that allows the electrodes to be spaced close together so that a high electric field can be established with smaller applied voltages.
When the operating frequency of the modulator is high, a travelling wave configuration can be introduced. The travelling wave configuration removes the frequency response limitation due to the RC constant and the transient time in a lumped circuit modulator. However, the frequency response will depend on the electrode RF loss, and the velocity matching between the optical wave in the waveguide and the electrical wave propagating in the RF transmission line.
The propagation constant of the fundamental mode in an optical waveguide is determined by the wavelength of the light beam, the index of refraction of the semiconductor material, and the geometry of the waveguide. The index of refraction of the waveguide is fixed by the specific semiconductor material being used. The propagation constant of the driving RF wave depends on the circuit structure of the transmission line. In general, a characteristic impedance Z0, propagation constant βe, and loss α of a transmission line can be expressed by:
            Z      0        =                  L        C              ,            B      e        =          Ω      ⁢              LC              ,            and      ⁢                          ⁢      α        =          R              2        ⁢                  Z          0                      ,
where L, C and R are the equivalent inductor, capacitor and resistor per unit length, respectively, and Σ is the frequency. The impedance of the transmission line is nearly fixed due to the need for matching to the driving circuit near 50Σ. Thus, the ratio of L over C is fixed. The expression of loss coefficient assumes that the metal loss is the dominating loss mechanism, although the doped region in the PIN structure can introduce significant loss as well. Therefore, R is inversely proportional to the width W of the top electrode. On the other hand, the dominating capacitance is at the PIN junction, and is proportional to W. Lower RF loss requires a large W, which in turn leads to a large C. Since the ratio of L/C is fixed due to the impedance consideration, it leads to a large βe, or a slow RF wave in the transmission line compared to the optical wave propagating in the optical waveguide.
The slow wave nature of the RF signal in the transmission line can limit the frequency response of the modulator, and prevent longer device structures for lower modulation voltages. This represents a difficult design trade-off between the microwave loss, which requires a wide electrode, and velocity matching, which requires a narrow electrode. A design approach that can improve the velocity mismatch in a semiconductor PIN modulator is advantageous since high RF loss is in general not acceptable in any device structure.
Previous attempts in the art to correct for velocity mismatch in a semiconductor PIN modulator have almost exclusively been directed to changing the electrode design, which changes the speed of the RF wave to match that of the optical wave. However, this type of approach has design trade-offs, as discussed above, and has only been successful so far, to the best knowledge of the authors, in further slowing down the microwave speed without introducing additional loss.
Velocity matching has been pursued in various modulator materials and structures. In LiNbO3 modulators, the microwave travels slower than the optical wave. Electrode designs that can reduce the distributed capacitance and therefore speed-up the RF wave have been demonstrated and widely used in commercial VMTW modulators. In bulk semiconductor modulators, the RF wave generally travels faster than the optical wave. Slow wave RF electrodes have been proposed and demonstrated for improved velocity matching. In both cases, modifications to the RF transmission lines have been performed to realize the velocity match, since the RF transmission lines are easier to maneuver due to large dimensions and freedom of physical layout.
What is needed is design approach that provides velocity matching between the RF wave and the optical wave in a semiconductor optical modulator without affecting the RF loss. It is therefore an object of the present invention to provide a design approach that accomplishes this velocity matching.